Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
=
Title.
Ultrasonic welding device.
Object of the invention.
The object of the present invention is an ultrasonic welding device,
particularly
for the welding of plastic parts. The device is comprised of a sonotrode, a
booster and a converter, suitable for attachment to a handler, actuator or
robotic
arm that moves it to the welding spot to proceed with the welding operation,
and
to which a holding and cooling down head is attached.
A main aspect of the invention is to provide the device with the proper means
to
apply frontal pressure to the parts to be welded, in a peripheral zone very
close
to the welding spot, in order to eliminate the unwanted gap between the parts;
and to effectively cool down the sonotrode and the weld once it is formed.
Field of application of the invention.
This invention is applicable in the welding of plastics by ultrasound.
State of the art
In several sectors, such as the automotive sector, ultrasound is commonly used
to weld plastics; one of the pieces to be joined has a set of bores, and studs
are
defined on the second part which pass through these bores. Both parts to be
welded are overlapped prior to being welded. During the welding operation, a
sonotrode moves in the axial direction against the hollow plastic stud,
causing
the plastic to melt and compress, forming a stud head or rivet on the first
part
which prevents the release of either part.
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Cassette machines equipped with several spot welding sonotrodes are used to
perform multiple welds simultaneously, providing a repetitive configuration.
Changing the parts to be welded requires a model change and the
reconfiguration of the machine for each model.
More flexible ultrasonic welding devices are also known, with robotic arms,
which allow the model to be changed by only modifying the coordinates.
During the use of these welding devices two problems usually arise:
= a first problem related to the proper cooling of sonotrode and the weld,
for their efficiency and;
= a second problem related to the correct positioning of the parts to
joined,
so that an unwanted gap is not left between them.
Related to the first problem, sonotrode cooling, there are several
antecedents:
In patent US3438428 a method is described for maintaining a vibratory tool at
a
controlled temperature. For its implementation, such a vibrating tool is
applied
to the plastic welding by means of a sonotrode. In order to carry out the
aforementioned temperature control, a conduit is defined in the sonotrode to
which the inlet and outlet of the external circulation circuit of a cooling
fluid are
connected.
Patent US6691909 refers to a device and a method for working parts in
ultrasonic welding which reduces sonotrode adhesion during the ultrasonic
welding process. Document US6691909 also proposes an internal cooling
system for the sonotrode, similar to that proposed in US3438428.
Document EP1000732 describes a device for the ultrasonic welding of elements
for resin fixing, and proposes forming a passage for the air defined across a
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portion of the sonotrode, one of whose uses is to inject air in order to cool
down
the welded area.
These antecedents have some disadvantages: the projection of air through an
internal conduit of the sonotrode can cause the appearance of bubbles or
filaments in the melted material and consequently defective welds;
furthermore,
this projection is not especially effective since the advance of the sonotrode
on
the molten material plugs the inner bore with molten plastic, preventing air
circulation inside the sonotrode and its exit at the tip or the front end, and
excessively increasing the temperature of the sonotrode.
In relation to the second problem of positioning of the parts to be welded,
the
use of external handlers, separate from the welding device, which hold the
parts
in a zone more or less distant from the welding area is also known
This often causes, especially in cases in which there is a material between
the
parts to be welded (e.g. a lining), the two parts to not be sufficiently held
together in the welding zone, so that the pieces do not stay together but have
an unwanted gap or groove between them, which in some cases this is
unacceptable to the manufacturer.
The applicant is not aware of the existence of antecedents regarding
ultrasonic
welding devices that could satisfactorily and simultaneously solve both
exposed
problems, regarding the sonotrode cooling, or the correct positioning of parts
in
the welding zones to avoid gaps or grooves being left between them.
Description of the invention
The ultrasonic welding device that is the object of the invention, comprised
of a
sonotrode, an ultrasonic converter and a booster, suitable for attachment to a
handler or robotic arm, and to which a holder for cooling down is attached,
with
characteristics suited to resolving the problems exposed, in particular, to
apply
a frontal pressure on the parts to be welded, in a peripheral zone very close
to
the welding spot, in order to eliminate the unwanted gap between the parts;
and
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to effectively cool the sonotrode, by its outer surface, guaranteeing the
circulation of pressurized air toward the welding zone during the performance
of
the weld.
For this purpose, and according to the invention, this device is comprised of
an
outer sleeve arranged around the sonotrode and which delimits, along with the
sonotrode, an annular chamber section for the circulation of pressurized air
which is connected, through an internal cavity of the head, to an inlet of
pressurized air, and which has an annular crenellated opening above, for the
outlet of pressurized air and the peripheral cooling of the sonotrode.
Said outer sleeve has a rear portion mounted on the unit head with the
possibility for axial movement between: - a forward position in which the
sleeve's front end protrudes frontally regarding the tip of the sonotrode;
making up a holder which exerts a configurable pressure, by means of
compressible means, against the parts to be welded, in a peripheral area close
to the welding point, during the approach of the sonotrode to the parts to be
welded and before the tip of the sonotrode establishes contact with the parts
to
be welded, and - a backward position, determined by the advance movement
of the sonotrode during the welding operation while the sleeve front end
continues pressing the parts to be welded, and in this backward position the
sleeve front end is positioned in a coplanar manner, or in a very close plane,
to
the sonotrode tip.
With these characteristics, it allows the front end of the sleeve to exert a
configurable pressure on the parts to be welded, in a peripheral area close to
the welding zone, before welding begins and throughout the welding process,
so the correct positioning of the parts remains guaranteed, preventing grooves
or undesired gaps between them.
The invention also allows the pressurized air to be used for the external
cooling
of the sonotrode, to circulate through the annular chamber and go out through
the front opening of the sleeve, even during the welding operation, due to the
crenellated configuration of the front opening of the sleeve which prevents
the
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molten plastic from blocking the exit of the compressed air, contrary to what
happens in the aforementioned antecedents that provide an exit through a
central bore defined in the sonotrode itself.
.. The pressurized cooling air provides an additional beneficial effect on the
performance on this kind of welding: when the sonotrode is removed after the
weld, that compressed air cools down the molten plastic on the weld point,
causing the solidification of the molten material, allowing the sonotrode to
be
removed without pulling on the molten material and, therefore, without
deforming and/or weakening the weld, which prevents having to dedicate the
time that is usually dedicated to waiting for the molten plastic to solidify¨
without refrigeration¨in order to be able to remove the sonotrode without the
troublesome deformations of the weld. The result of this effect is that welds
are
carried out at shorter intervals; meaning with higher throughput per unit.
Furthermore, the pressure exerted on the parts remains effective in the
backward motion of the sonotrode, thereby preventing separation while cooling
the weld with the pressurized air.
An additional advantage of circulating pressurized air through the inner
cavity of
the device is that the air pressure itself causes a self-centring effect in
the radial
direction of the sleeve with regard to the mechanical guiding element, thus
minimizing wear by mechanical friction between them, since it only acts in the
event of extreme radial forces.
Description of the figures.
In order to complement the description being made, and in order to facilitate
.. the comprehension of the characteristics of the invention, a set of
drawings is
attached to this specification in which, in an illustrative and non-limiting
nature,
the following has been represented:
- Figure 1 shows an elevated cross-section view of an example of the
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implementation of the ultrasonic welding device according to the invention, in
an inoperative position and in which the sleeve can be seen in the forward
position.
- Figure 2 shows the pushing and cooling head in a view similar to the
previous one in which the ultrasonic welding elements have been removed.
- Figure 3 shows an elevated cross-section view of the ultrasonic welding
device of Figure 1 in an operating position, with the sleeve in a backward
position.
- Figures 4 and 5 show both details of a front portion of the invented device
in
different phases of the welding process of two pieces of plastic material.
Preferred implementation of the invention.
The ultrasonic welding device shown on figures 1 and 3, as an example of the
implementation, is comprised of a sonotrode (1) attached by its rear area to a
booster (12) and this in turn to a converter (13) suitable for its connection
to an
actuator, handler or robotic arm responsible for its movement, and to which
the
head (2) is attached.
The device is comprised of an outer sleeve (3) arranged around the sonotrode
(1) and which delimits, along with the sonotrode, an annular chamber (4) for
the circulation of pressurized air connected through the cavity (23) to an
inlet
(21) of pressurized air defined in the head (2); providing the outer sleeve
(3) at
its front end (31) with a crenellated mouth for the exit of pressurized air at
the
periphery of the tip (11) of the sonotrode (1) and the peripheral cooling of
this
sonotrode (1).
The outer sleeve (3) has a rear portion (32) assembled in the head (2) with
possibility for axial movement, between a forward position shown in figure 1,
and a backward position shown in the figure 3.
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In the forward position of the sleeve (3) the front end (31) protrudes
frontally
regarding the tip (11) of the sonotrode (1), whereas in the backward position
the front end (31) of the sleeve is provided in a coplanar manner or in a
plane
close to the tip (11) of the sonotrode (1).
The device is comprised of a support (5) attached externally to an
intermediate
area of the sleeve (3) and it moves along with this sleeve (3) with regard to
the
head (2), between the aforementioned forward and backward positions.
The device is comprised of compressible means (51) represented in this
example by some helical springs which act on the support (5) and tend to hold
it in the forward position, in contact with a mechanical stop (22) fixed to
the
head (2) and which limits the advance of the support (5) in the forward
position, as shown in figure 1.
The rear portion (32) of the outer sleeve (3) is housed in the chamber (23) of
the head (2) which is conveyed through a perforated cylindrical bushing (6)
with the lateral pressurized air inlet (21) defined in the aforementioned head
(2); this rear portion of the sleeve (32) is comprised of a crenellated back
(33)
and some radial bores (34) for the passage of pressurized air from the
chamber (23) of the head (2) toward the annular chamber (4) defined between
the external sleeve (3) and the sonotrode (1), regardless the more or less
advanced position of the this outer sleeve (3).
The perforated cylindrical bushing (6) is housed in a peripheral seat (24)
defined in the chamber (23) of the head (2) and fixed in this peripheral seat
(24) by means of a retainer (7) fixed to the head (2) which has a gasket (71)
that acts against the movable support (5) preventing pressurized air from
leaking through the space between the support (5) and the head (2).
Figures 4 and 5 represent two positions of the device during operation in an
axial direction and the welding of two plastic parts (P1, P2) conveniently
superimposed and provided respectively with a stud (T) and a bore (0) for the
passage of the stud (T).
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As shown on figure 4, during the approach of the sonotrode (1) to the parts
(P1, P2) to be welded, before the tip (11) of the sonotrode (1) establishes
contact with the parts to be welded, the sleeve (3) is positioned protruding
forward and its front end (31) forms a pusher that exerts, through the action
of
the compressible means (51), a configurable pressure against the parts (P1,
P2) to be welded, in a peripheral zone close to the weld, immobilizing them in
the suitable position and preventing any gap or groove from remaining
between them..
As shown in figure 5, the sonotrode tip (11) acts against the stud (T) during
the
forward movement of the device in an axial direction, performing the weld; and
the front end of the sleeve (31) held the parts to be welded (P1, P2) pressed
together, reaching a backward position, precisely determined by this forward
movement of the sonotrode (1).
The sleeve (3) moves back with regard to the sonotrode (1) during the
welding, causing the compression of the compressible means (51) which tend
to maintain it in the former position.
Once the nature of the invention has been sufficiently described, along with
an
example of its preferred implementation, it should be stated to the
appropriate
effects that the materials, shape, size and arrangement of the described
elements may be modified, provided that this does not imply an alteration of
the essential characteristics of the invention which are claimed below.
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